1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device which includes a nitride group functional semiconductor layer having a two-dimensional carrier gas layer near a hetero junction.
2. Description of the Related Art
Horizontal Shottky barrier diodes (SBD) are well-known, and have a hetero junction structure constituted by gallium nitride (GaN) and aluminum gallium nitride (AlGaN). Such a horizontal SBD is fabricated as a nitride group functional semiconductor layer having a GaN layer and an AlGaN layer placed on the GaN layer by the hetero junction. The SBD also includes not only a two-dimensional electron gas channel layer (2EDG) which is generated near the hetero junction of the GaN layer but also an anode electrode and a cathode electrode which are electrically connected to opposite ends of the 2EDG channel layer by the tunnel effect or the like.
The anode electrode is electrically connected to the one end of the 2EDG channel layer by forming a Shottky barrier, and is a composite film of a nickel (Ni) layer and a gold (Au) layer on the Ni layer, for instance.
Further, the cathode electrode is in ohmic contact with the other end of the 2EDG channel layer, and is a composite layer of a titanium (Ti) layer and an aluminum (Al) layer on the titanium layer.
An example of the horizontal SBDs is disclosed in WO 03/071607 A1.
The invention disclosed in the foregoing publication seems to have the following problems. With the horizontal SBD, the anode electrode is constituted by a composite film of the Ni and Au layers, and has a large reverse leakage current. The large reverse leakage current can be reduced by selectively using a transition metal such as platinum (Pt) or rhodium (Rh) as an electrode material for the anode electrode, but cannot be suppressed sufficiently. If such a material is used for the anode electrode, the horizontal SBD tends to have a high rising voltage Vf during a forward operation. Further, the horizontal SBD has a high differential on-resistance.
In terms of the crystal growth of the nitride group semiconductor functional layer, it is conceivable to reduce a carrier density of the secondary nitride group semiconductor functional layer produced near the hetero junction, and to suppress the reverse leakage current. However, as the carrier density is reduced, the differential on-state resistance tends to become higher in the forward operation of existing horizontal SBDs. In other words, the reduction of the reverse leakage current and the reduction of the differential on-resistance are contradictory to each other.